The flow of air adjacent to the surface of an aircraft wing or component, i.e., the so-called boundary-layer, can be either laminar or turbulent. The onset of turbulence brings about substantial increases in such aerodynamic parameters as drag and heat transfer. It is therefore important to study the instability waves which precede turbulence, and perhaps to try to obtain a feedback system to reduce the ill effects associated with turbulence.
Boundary-layer turbulence in general is initiated by instability waves, which are ever-present in laminar layers. Due to the effects they produce, such waves are of considerable importance for both scientific and engineering reasons.
Aircraft wing design procedures have for several years included computer programs which predict the growth of waves, and the resultant onset of turbulence. The computations are far from perfect, as is the understanding upon which they are based.
An improved method for instability wave detection and measurement is needed. Such a method could be applied to instability wave suppression for turbulence control on aerodynamic surfaces, whereupon aircraft drag would be reduced.
The most commonly employed method for wave-detection in the research laboratory is hot-wire anemometry, but this technique has several drawbacks. Hot-wire probes are invasive and easily broken. When using hot-wire probes it is necessary to survey across the boundary layer thickness to find the maximum value of the wave amplitude. Hot-wire probes and their electronic control units are quite costly. Finally, the intrusiveness of hot-wire probes itself causes turbulent waves which preclude observation at more downwind stations.